Recently, organic EL devices have attracted attention as light-emitting displays alternative to liquid crystal displays. Organic EL devices according to prior arts utilize emission (fluorescence) from a singlet excited state. In such a case, a local maximum emission efficiency is 25% based on a principle of an organic EL phenomenon, and therefore the emission efficiency is extremely low.
As a method of increasing emission efficiency, phosphorescence generated from a triplet excited state has attracted particular attention recently (see Non-Patent Document 1, for example).
In such a case, the emission efficiency may be 100% in theory.
Many PtII complexes having diimine or terpyridine and their derivatives exhibit emission which are assigned to MLCT (abbreviation of metal-to-ligand charge transfer; charge transfer from a metal ion to a ligand) or MMLCT (abbreviation of metal-metal-to-ligand charge transfer; charge transfer from a dσ* orbital formed by metal-metal interaction to a ligand), and photophysical properties of these compounds have attracted much interest (See Non-Patent Document 2, for example).
Polynuclear CuI and AuI complexes of pyrazole and its derivatives are also known to exhibit emission (see Non-Patent Document 3, for example).
Accordingly, when a molecule is synthesized with PtII ions and CuI ions, AgI ions or AuI ions and these metal ions are bridged by pyrazole or its derivatives, it is promising to produce a new molecule having emission properties by a synergetic effect of different metal ions.
In development of a novel metal complex based on this idea, a mixed metal complex [Pd2Ag4(μ-dmpz)8] having two PdII ions and four AgI ions bridged by 3,5-dimethylpyrazolate (denoted as dmpz, which is a monovalent anion in which a proton is dissociated from 3,5-dimethylpyrazole (dmpzH)) is known (see Non-Patent Document 4); however, emission properties of this compound have never been reported.
The present inventors also have already synthesized a mixed metal complex [Pt2Ag4(μ-pz)8] having PtII ions and AgI ions bridged by pyrazolate by using pyrazole (denoted as pz, which is a monovalent anion in which a proton is dissociated from pyrazole (pzH)) without substituent groups (see Non-Patent Document 5); however, this compound does not show emission.
Furthermore, in development of displays for commercialization, there is an increasing demand for a novel metal complex used as a dopant having improved thermal stability, volatility, film-forming properties during deposition, solubility in various solvents, emission intensity, color purity, and stability when applying a potential.    [Non-Patent Document 1] M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4-6.    [Non-Patent Document 2] S.-W. Lai, C.-M. Che, Topics in Current Chemistry, 2004, 241(Transition Metal and Rare Earth Compounds III), 27-63.    [Non-Patent Document 3] H. V. R. Dias, H. V. K. Diyabalanage, M. G. Eldabaja, O. Elbjeirami, M. A. Rawashdeh-Omary, M. A. Omary, J. Am. Chem. Soc., 2005, 127, 7489-7501.    [Non-Patent Document 4] G. A. Ardizzoia, G. La Monica, S. Cenini, M. Moret, N. Masciocchi, J. Chem. Soc., Dalton Trans. 1996, 1351-1357.    [Non-Patent Document 5] K. Umakoshi, Y. Yamauchi, K. Nakamiya, T. Kojima, M. Yamasaki, H. Kawano, M. Onishi, Inorg. Chem. 2003, 42, 3907-3916.